Downhole tools having circumferentially spaced rolling elements

ABSTRACT

A downhole tool for providing rotary support of a downhole assembly in which the tool is incorporated, the tool also converting rotary contact with the wellbore to a longitudinal force tending to propel the assembly along the wellbore. The tool resembles a roller stabilizer in which the roller axes are skewed to be tangential to a notional helix, such that the natural (non-slipping) paths of roller contact with the wellbore have a longitudinal component in addition to the usual circumferential path. The tool can be used on drill strings and in downhole motor assemblies. The invention has particular advantage in highly deviated wells since it simultaneously compensates for increased bore friction and dynamically enhances weight-on-bit.

This invention relates to downhole tools, and relates more particularlybut not exclusively to downhole tools in the form of well-drilling toolswhich facilitate the drilling of wells which are substantiallynon-vertical.

BACKGROUND

As oil and gas reserves become scarcer or depleted, methods for moreefficient production have to be developed.

In recent years horizontal drilling has proved to enhance greatly therate of production from wells producing in tight or depleted formation.Tight formations typically are hydrocarbon-bearing formations with poorpermeability, such as the Austin Chalk in the United States and theDanian Chalk in the Danish Sector of the North Sea.

In these tight formations oil production rates have dropped rapidly whenconventional wells have been drilled. This is due to the small sectionof producing formation open to the well bore.

However when the well bore has been drilled horizontally through the oilproducing zones, the producing section of the hole is greatly extendedresulting in dramatic increases in production. This has also proved tobe effective in depleted formations which have been produced for someyears and have dropped in production output.

However, horizontal drilling has many inherent difficulties. In broadterms the difficulties include the following factors:

(i) the rotational torque requirement of the drillstring rises rapidlywith increasing hole angle (angular displacement from vertical) andlength of the horizontal section,

(ii) the weight of the drillstring in the vertical section of the holemust push the drillpipe along the horizontal section thereby increasingthe fatigue stresses in the drillpipe located on the bend between thetwo sections,

and

(iii) performance of the drillbit is reduced due to both (i) and (ii)above as difficulties in applying weight and torque affect the ROP("rate of progress" in deepening/lengthening of the well).

PRIOR ART

Conventional stabilisers used in assemblies for horizontal drilling dolittle to resolve the above problems. Conventional stabilisers havefixed blades which normally are spiralled to distribute well contactarea whilst still allowing fluid bypass. Conventional stabilisers alsogenerate quite considerable back torque and resistance to forward motionalthough they do centralise the drilling assembly and play an importantrole in directional control of the hole.

A number of attempts have been made to reduce friction by thedevelopment of rolling element stabilisers. A recent one of thesestabiliser tools (described in published European Patent ApplicationEP0333450-A1) used freely rotating balls set into the stabiliser bladeswhich addressed points (i) and (ii) above. initially the tool was wellreceived by the oil industry as there was a real need to resolve thedownhole torque problems. Unfortunately the tool proved to have problemswith the balls packing off and locking with cutting debris. Thisconsiderably reduced the market interest in this tool.

Another known form of rolling element stabiliser is based on rollersmounted on respective axes which are each parallel to the longitudinalaxis of the stabiliser and hence parallel to the longitudinal axis ofthe drillstring and of the well drilled thereby. Examples of this formof roller stabiliser are described in U.S. Pat. 3907048 and UnitedKingdom Patent Specification GB271839. The functional effect of thisform of roller stabiliser is to reduce rotational friction (by reason ofthe rolling support of the stabiliser against the bore of the well orwell casing), but to have a neutral longitudinal effect (by reason ofthe parallelism of the roller axes with respect to the longitudinal axisof the stabiliser and the drillstring incorporating the stabiliser).

A still further form of rolling element stabiliser which purports toreduce both rotational and longitudinal friction is described in U.S.Pat. No. 1,913,365. This further form of roller stabiliser essentiallycomprises a collar which is rotatably mounted on the exterior of adrillstring by two rows of vertical-axis rollers, i.e. rollers whoserespective axes are each parallel to and radially offset from thelongitudinal axis of the drillstring. (These vertical-axis rollers areexternally spherically shaped, and therefore superficially appear asballs, although they are actually rollers). While the collar is free torotate on the drillstring (by reason of the rolling support provided bythe vertical-axis rollers), the collar is longitudinally retained at afixed position on the drillstring by end rings clamped to thedrillstring. The collar provides longitudinal rolling support for thedrillstring by means of an external array of horizontal-axis rollers,i.e. rollers whose respective axes are each tangential to a circlecentered on the longitudinal axis of the drillstring. Thus although thisfurther form of roller stabiliser provides both rotational andlongitudinal rolling support for the drillstring, it is to be noted thatthe purely longitudinal ("vertical") and circumferential ("horizontal")roller axes result in the facts that rotational movement of thedrillstring does not result in a net longitudinal force, nor doeslongitudinal movement of the drillstring result in a net rotationalforce, i.e. there is no cross-translation of motion and force betweenrotational and longitudinal directions.

U.S. Pat. No. 4,000,783 describes a roller reamer, i.e. a form ofannular drilling bit for substantially enlarging the bore of a pilothole. In this roller reamer, the conical reamers or cutters arerotatably mounted on respective axes that are each triply offset fromthe longitudinal axis of the drillstring, being offset radiallyoutwards, obliquely (i.e. conically), and skewed (i.e. helical) withrespect to the drillstring axis. The conical reamers enlarge apreviously-drilled hole by gouging away the wall of the pilot hole in anannular region around the tool. It is said that if the reamers aredisposed at a skew angle which is greater than the neutral skew angle,the cutters provide a self-advancing action. It is to be noted that theconical reamers or cutters of U.S. 4,000,783 provide a purely cuttingaction, with radial support of this cutting tool being provided bypurely static cylindrical shoulders ahead of and behind the cutters (seeFIG. 1 of U.S. Pat. No. 4,000,783), a smaller diameter shoulderproviding radial support in the pilot hole, and a larger diametershoulder providing radial support in the enlarged bore. These radialsupport shoulders are concentric with the longitudinal axis of the tooland of the drillstring.

OBJECTS OF THE INVENTION

It is an object of the invention to provide a downhole tool whichprovides radial support for a rotatable downhole assembly in apreviously drilled hole of substantially uniform diameter, the radialsupport being provided by a rolling element arrangement which translatesrotational movement of the tool to a longitudinal force on the tool.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided adownhole tool for providing radial support for a rotatable downholeassembly within a previously drilled hole of substantially uniformdiameter, said tool comprising a central member constructed or adaptedto be incorporated in a rotatable downhole assembly for rotationtherewith in use of said tool, said central member mounting a pluralityof rolling element means in respective positions which arecircumferentially distributed around said tool, each said rollingelement means being rotatably mounted on a respective axis which istangential to a notional helix substantially coaxial with thelongitudinal axis of said tool about which said tool rotates in use ofsaid tool such that each said respective axis of said rolling elementmeans is skewed with respect to said longitudinal axis, each saidrolling element means having a respective periphery which extends to theradially outermost periphery of said tool whereby the radially outermostperiphery of said tool provides rolling radial support for saidrotatable downhole assembly in use of said tool by means of theperipheries of said rolling element means and the rotation of saidrolling element means about their skewed axes translates rotation ofsaid tool in use thereof to a longitudinally-directed force actingthrough said central member on said downhole assembly.

Said rotatable downhole assembly may be a drillstring and said notionalhelix is preferably contra-rotary with respect to the combination of thenormal or forward direction of rotation of the drillstring and thedirection from said tool towards a drill bit at the downhole end of thedrillstring, whereby normal or forward rotation of said drillstring andof the tool incorporated therein results in a longitudinal force tendingto propel the drillstring towards the blind end of the bore andultimately tending to force the drill bit into the geological materialto be drilled. Thus if the normal or forward direction of rotation ofthe drillstring is clockwise as viewed from the surface and looking downinto the bore, said notional helix preferably progresses anti-clockwisein a downhole direction therealong whereby the peripheries of saidrolling element means, where they extend to the radially outermostperiphery of the tool, align with a notional right-hand thread aroundthe outer periphery of said tool.

Each respective axis of said rolling element means is preferably skewedwith repict to the longitudinal axis of the tool at an angle in therange from a very low (but non-zero) angle, up to 45°, and morepreferably at an angle in the range from 0.5° to 15°. Said downhole toolmay incorporate skew angle variation means operable to make the skewangle controllably variable, and possibly capable of reversing the handof said notional helix whereby the direction of longitudinal force isreversed without reversing the direction of rotation.

Said rolling element means are preferably rollers, and the peripheriesof said rollers may individually be cylindrical or crowned (i.e. havingrelatively larger diameter mid-length portion reducing continuously ordiscontinuously to a relatively smaller diameter at either end). Saidrollers may be individually mounted on a respective axis, or saidrollers may be mounted in coaxial groups, preferably such that within agroup of rollers, individual rollers of that group are capable ofrotating at mutually differing rotational rates.

Radial force applying means are preferably incorporated in the tool forapplying radially outwardly directed radial forces to the rollingelement means to increase their traction on the bore. The radial forceapplying means may be such that the radially outwardly directed radialforces applied to the rolling element means are controllably variable.

The central member of the tool may be adapted from a conventionalfixed-blade stabiliser by reducing the outside diameter slightly belowthe nominal diameter of the bore of the well in which the tool is to beused, machining or otherwise forming pockets or recesses in the blades,and mounting a roller assembly in each of these pockets or recesses suchthat the rollers project to define the gauge or radially outermostperiphery of the tool at the nominal well bore diameter. Each rollerassembly can comprise a single roller or a group of rollers mounted onan axle which is rotatably mounted at each end thereof by a suitablecombination of radial bearings and thrust bearings.

According to a second aspect of the present invention there is provideda rotatable downhole assembly for rotatable operation within apreviously drilled hole of substantially uniform diameter, said downholeassembly comprising a downhole motor having a motor housing and arotatable motor output shaft coupled to a rotatable motor outpututilisation means, said downhole assembly further comprising at leastone downhole tool according to the first aspect of the presentinvention, said at least one downhole tool being coupled between saidrotatable motor output shaft and said rotatable motor output utilisationmeans for rotation therewith in operation of said assembly to provideradial support therefor and to translate such rotation to alongitudinally-directed force acting through said motor outpututilisation means.

Said downhole assembly may comprise a plurality of such downhole tools,each according to the first aspect of the present invention, and eachbeing coupled between said rotatable motor output shaft and saidrotatable motor output utilisation means, said tools being optionallymutually separated by one or more drill collars or other suitablelongitudinal spacer means serving in operation of said assembly toconvey torque, rotation, and longitudinal forces between parts of saidassembly mutually separated by such spacer means.

Said rotatable motor output utilisation means may comprise a drill bit,said at least one downhole tool comprised in said downhole assemblybeing formed dynamically to increase the effective weight-on-bit duringnormally directed rotation of said drill bit by said downhole motor.

Said motor housing is preferably coupled to countertorque means forreacting motor torque output by said motor output shaft, saidcountertorque means rotationally constraining said motor housing withrespect to said previously drilled hole. Said countertorque means mayprovide a rotational braking effect while allowing relative freedom ofmovement in a longitudinal direction, preferably by forming saidcountertorque means with a peripheral array of hole-contacting rotatablerollers having their axes of rotation substantially tangential tonotional circles substantially coaxial with the longitudinal axis ofsaid downhole assembly. Alternatively, said countertorque means maycomprise a further downhole tool in accordance with the first aspect ofthe present invention, the notional helix of said further downhole toolbeing oppositely handed with respect to the notional helix of said atleast one downhole tool coupled between said rotatable motor outputshaft and said rotatable motor output utilisation means whereby relativecontrarotation of said motor housing with respect to said motor outputshaft results in commonly directed longitudinal forces at said at leastone and further downhole tools comprised in said downhole assembly.

The motor of said downhole assembly may be a hydraulic motor supplied inoperation thereof with pressurised fluid by way of tubing which may beflexible (i.e., tubing which is known in the art as "coiled tubing"),said downhole assembly preferably being coupled to said tubing by way ofa swivel coupling which is preferably substantially fluid-tight.

Said downhole assembly may have major components and sub-assembliesthereof longitudinally coupled by one or more couplings transmissive oftorque and longitudinal forces but yieldable about axes transverse tothe longitudinal axis whereby the downhole assembly may conform to bentholes.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Embodiments of the present invention will now be described by way ofexample, with reference to the accompanying drawings wherein:

FIG. 1 is an elevational view of a first embodiment of the presentinvention;

FIG. 2 is an elevational view of a form of roller suitable for use withthe present invention;

FIG. 3 is an elevational view of another form of roller suitable for usewith the present invention;

FIGS. 4 and 5 are respectively an elevational view and a plan view of asecond embodiment of the present invention;

FIG. 6 and 7 are respectively an elevational view and a plan view of athird embodiment of the present invention;

FIG. 8 is an elevational view of a fourth embodiment of the presentinvention;

FIG. 9 is a schematic longitudinal elevation of a fifth embodiment ofthe present invention;

FIG. 10 is a schematic longitudinal elevation of a sixth embodiment ofthe present invention;

FIG. 11 is a schematic longitudinal elevation of a seventh embodiment ofthe present invention;

FIGS. 12 and 13 are respectively an elevational view and a plan view ofan eighth embodiment of the present invention;

FIG. 14 is a schematic longitudinal elevation of a ninth embodiment ofthe present invention;

FIGS. 15 and 16 are elevational views of a tenth embodiment of thepresent invention, taken in mutually orthogonal directions;

FIG. 17 is a perspective view of an eleventh embodiment of the presentinvention;

FIGS. 18 and 19 are respectively schematic elevational and plan views ofa twelfth embodiment of the present invention; and

FIGS. 20 and 21 are respectively schematic elevational and plan views ofa thirteenth embodiment of the present invention.

Referring first to FIG. 1, a first embodiment of downhole tool 10 inaccordance with the present invention comprises a central member 12whose form is generally that of a conventional fixed-blade stabiliser.The central member 12 comprises a hollow shaft 14 having a standardA.P.I. (American Petroleum Institute) box connector 16 at the upper endand a standard A.P.I. pin connector 18 at the lower end for connectionof the tool 10 in a conventional drillstring (not shown).

The shaft 14 of the central member 12 has three spiral blades 20 formedintegrally thereon, each of the blades 20 describing a clockwise helix.The radially outer edge 22 of each blade 20 has a radius (measured fromthe longitudinal axis of the tool 10) which is slightly less than thenominal gauge of the tool 10, i.e. a radius slightly less than theradius of the bore in which the tool 10 is designed to be used.

Three pockets 24 are cut through each outer edge 22 and into the bodiesof the blades 20. Within each pocket 24, a roller 26 is rotatablymounted on a respective axle 28 such that part of the outer periphery ofeach roller 26 radially extends beyond the respective outer edge 22 ofthe respective blade 20 to define the radially outermost periphery ofthe tool 10.

Each of the roller axles 25 is skewed with respect to the longitudinalaxis of the tool 10 about which the tool 10 rotates in use thereof, i.e.each roller axle 28 is tangential to a respective notional helixsubstantially coaxial with the longitudinal axis of the tool 10 andspiralling anti-clockwise in a downward direction (i.e. each notionalhelix is of opposite hand to the illustrated helical shape of the blades20). As shown in FIG. 1, the roller axles 28 extend transversely of theblades 20, and therefore a notional point on the outer periphery of anyone of the rollers 26 would, as the roller rotated and where thenotional point was proud of the respective blade 20, describe a pathgenerally along the line of the outer edge 22 of that blade, i.e. anotional right-hand thread around the outer periphery of the tool 10.

The result of this roller mounting configuration is that the array ofrollers 26 provides rolling support for the tool 10, and hence for thedrillstring in which it is incorporated, by bearing against thesubstantially uniform diameter bore of the hole drilled by the drillingbit above which the tool 20 is fitted, while simultaneously reactingwith the bore to translate the clockwise rotation of the tool 10 (asviewed from above and looking downhole) into a downwardly-directedlongitudinal force by reason of the skewing of each roller 26 asdescribed above. Thus, in normal drilling operations while thedrillstring is rotating clockwise (as viewed from above and lookingdownhole), the tool 10 will cause the drillstring to "walk" downhole, soenhancing the pressure on the drill bit and improving ROP. Thisbeneficial and desirable effect is enhanced by increased side-loading onthe tool 10, such as will be experienced as the bore increasinglydeviates from vertical, to reach a maximum in horizontal stretches ofthe bore (where the weight of the horizontal sections of the drillstringis ineffective to push the drill bit forwards). It is also in suchdeviated and ultimately horizontal stretches of the bore thatlow-friction radial support of the drillstring is most required, and isprovided by the tool 10 simultaneously with the above-described tractiveeffort.

The skew angle at which each of the rollers 26 is mounted on the tool 10may be any non-zero angle from a very small angle (e.g., under 1°) up toabout 45° (or greater in appropriate circumstances), and is preferablyin the range 0.5°-15°. The skew angle is preferably selected to give agreater rate of theoretical progress (as denoted by the pitch of theabove-mentioned notional thread) than the maximum ROP practicallyachievable by the drill bit, such that there is always a forward(downhole-directed) tractive effort during forward (clockwise) rotationof the drillstring.

As is clearly shown in FIG. 1, the rollers 26 are angularly distributedaround the periphery of the tool 10, thus tending to give a relativelyuniform loading on the bore of the well in which the tool 10 is beingused. It should be noted that the well bore will necessarily be of asubstantially uniform diameter in those parts of the bore in which thetool 10 is used, since the tool 10 is devoid of any cutting, chiselling,reaming, or gouging action. Indeed, any such reaming action isundesirable, and is avoided at least partly by the suitable distributionof the rollers 26 and by the form of their peripheries (of which moredetails are given below).

Reversal of the direction of rotation of the drillstring (i.e. rotationof the drillstring in an anti-clockwise direction as viewed from aboveand looking downhole) will result in concomitant reversal of theabove-described longitudinal force to give an uphole-directed tractiveeffort which will assist in withdrawal of the drillstring from the well.Nevertheless, the desirable low-friction radial support of thedrillstring provided by the tool 10 incorporated therein will bemaintained even during such reverse rotation.

Referring now to FIGS. 2 and 3, these show two forms of roller suitablefor use in the present invention. In FIG. 2, the roller 200 is a crownroller having a (schematically depicted) rotation axis 202. The diameterof the roller periphery 204 varies smoothly (continuously) from amaximum at the mid-length to a somewhat lesser diameter at each end. Thelength of the roller 200 (as measured along its rotation axis 202) issimilar to the maximum diameter of its periphery 204. Crowning of theroller periphery 204 enhances distribution of the loading on the roller200 in its contact with the bore of the well, as does avoidance ofdiscontinuous changes in peripheral diameter.

In FIG. 3, the roller 300 is a barrel roller having a schematicallydepicted rotation axis 302. The roller periphery 304 has a mid-lengthsection 306 of substantially constant diameter which merges intoconically tapering sections 308 at each end of the roller 300. Thelength Of the roller 300 (as measured along its rotation axis 302) is asmall multiple of the maximum diameter of its periphery 304 (i.e. thediameter of the mid-length periphery section 306).

Referring now to FIGS. 4 and 5, these respectively illustrate anelevation and a plan view of a second embodiment of downhole tool 410 inaccordance with the present invention. The tool 410 is generally similarto the tool 10 previously described with reference to FIG. 1, andaccordingly those parts of the tool 410 which are identical orequivalent to parts of the tool 10 will be given the same referencenumerals, but preceded by a "4" (i.e. the FIG. 1 reference numerals plus400). The following description will concentrate principally on thoseparts of the tool 410 which differ from the tool 10, and for a detaileddescription of parts of the tool 410 not described below, referenceshould be made to the relevant parts of the foregoing description of thetool 10.

Apart from some differences in dimensional proportions (principally anincrease in relative lengths), the major difference in the tool 410 withrespect to the tool 10 lies in a substantial increase in the numbers ofrollers mounted in the periphery of the tool 410. As shown in FIG. 4, acorrespondingly increased number of pockets 424 is cut through eachouter edge 422 and into the bodies of the blades 420. The rollersmounted one in each of the pockets 424 are omitted from FIGS. 4 and 5,but are similar to the rollers 26 in the tool 10 as shown in FIG. 1; inparticular the skewing of the roller axles in the tool 410 isessentially the same as in the tool 10. The performance and functions ofthe tool 410 are as described above in respect of the tool 10, save forthe effects of the increased number of rollers.

Referring now to FIGS. 6 and 7, these respectively illustrate anelevation and a plan view of a third embodiment of downhole tool 510 inaccordance with the present invention. The tool 510 is similar to thetool 410 described above with reference to FIGS. 4 and 5, andaccordingly those parts of the tool 510 which are identical orequivalent to parts of the tool 410 will be given the same referencenumerals, but with the leading "4" substituted by a "5". The followingdescription will concentrate principally on those parts of the tool 510which differ from the tool 410, and for a detailed description of partsof the tool 510 not described below, reference should be made to therelevant parts of the foregoing descriptions of the tools 410 and 10.

The major difference in the tool 510 with respect to the tool 410 liesin the replacement of the crown rollers of the second embodiment with amuch increased number of needle rollers. Accordingly, the approximatelycircular roller pockets 424 of the second embodiment are replaced by acorrespondingly greater number of relatively narrow roller pockets 524cut through each outer edge 522 and into the bodies of the blades 520.The needle rollers mounted one in each of the pockets 524 are omittedfrom FIGS. 6 and 7, but are mounted with their rotation axis eachtransverse the respective blade 520. Because of the relatively smalldiameter and relatively great length/diameter ratio of the needlerollers of the third embodiment, it is preferred to mount the needlerollers each in a suitably re-entrant pocket, preferably lined with asuitable bearing material, to retain the rollers in the tool 510, ratherthan to mount the rollers on individual axles as in the otherembodiments of the present invention. Nevertheless, the rotationalalignment of each of the needle rollers of the third embodiment isessentially the same as for the rollers of the other embodiments. Theperformance and function of the tool 510 is the same described above inrespect of the tools 10 and 410, save for the effects of the number,size, and shape of the needle rollers.

Turning now to FIG. 8, this illustrates a downhole tool 610 which is afourth embodiment of the present invention. The tool 610 comprises acentral member 612 which has the form of a fixed-blade stabiliser with ahollow shaft 614 having a standard A.P.I. box connector 616 at the upperend, and a standard A.P.I. pin connector 618 at the lower end forconnection of the tool 610 in a conventional drillstring (not shown).

The shaft 614 of the central member 612 has three spiral blades 620formed integrally thereon, each of the blades 620 describing ananti-clockwise helix or left-handed spiral. (This is in contrast to theblades 20 in the tool 10, which each describe a clockwise helix orright-handed spiral). The radially outer edge 622 of each blade 620 hasa radius (measured from the longitudinal axis of the tool 610) which isslightly less than the nominal gauge of the tool 610, i.e. a radiusslightly less than the radius of the bore in which the tool 610 isdesigned to be used.

A recess 624 is cut from the outer edge 622 and into the body of eachblade 620. Within each pocket 624, a roller assembly 626 is rotatablymounted on a respective axle 628 such that part of the outer peripheryof each roller assembly 626 radially extends beyond the respective outeredge 622 of the respective blade 620 to define the radially outermostperiphery of the tool 610.

Each of the roller assembly axles 628 is skewed with respect to thelongitudinal axis of the tool 610 about which the tool 610 rotates inuse thereof, i.e. each roller assembly axle 628 is tangential to arespective notional helix substantially coaxial with the longitudinalaxis of the tool 610 and spiralling anti-clockwise in a downwarddirection (i.e. each notional helix is of the same hand as theillustrated helical shape of the blades 620, and in a preferred form ofthe fourth embodiment, each notional helix is substantially coincidentwith the center-line of the respective helical blade 620). As shown inFIG. 8, the roller assembly axles 628 extend longitudinally of theblades 620, and therefore a notional point in the outer periphery of anyone of the roller assemblies 626 would, as the roller assembly rotatedand where the notional point was proud of the respective blade 620,describe a path generally transverse the outer edge 622 of that blade,i.e. a notional right-hand thread around the outer periphery of the tool610.

Each of the roller assemblies 626 comprises a group of rollers 630coaxially mounted side-by-side along the respective axle 628 such thateach roller 630 can individually rotate independently of its neighbors,thereby permitting traction without slippage due to differentialrotational velocities along the roller assembly 626. The overall profileof each roller assembly 626 is ellipsoidal or hyperboloidal to suit thecircumferential curvature of the well bore in which the tool 610 isused, in conjunction with the selected skew angle of the axles 628 (thisskew angle preferably being in the range 0.5°-15°, and possibly up toabout 45°). End sections 632 of the roller assemblies 626 may beperipherally faced with wear-resisting inserts 634 (e.g. of tungstencarbide).

Opposite ends of each roller assembly axle 628 are housed in uncutawayportions of the body of the respective blade 620 wherein radial loadingon the respective axle 628 is sustained by radial bearings, and axialloading is sustained by suitable axial bearings. In order to give accessto a longitudinal axle-accommodating bore through the body of each blade620 from the lower end face thereof, the shaft 614 of the central member612 is made in two parts mutually connected by a standard A.P.I. pin andbox connector 636 (shown in ghost outline) joining the two shaft partsimmediately below the lower end faces of the blades 620.

Each roller assembly axle bearing arrangement may be provided with apressure-compensated grease reservoir 638 (only one being visible inFIG. 8) to provide lubrication therefor in a manner which inhibits theingress of drilling debris and other foreign material.

The portions of the blade edges 622 not cut away to form the rollerassembly recesses 624 may be faced with wear-resisting inserts 640 (e.g.of tungsten carbide) to mitigate the effects of unintended directcontact of the blade edges 622 with the well bore, such as may occur inthe event of excessive wear of the roller assemblies 626 or collapse oftheir axles 628 or of their bearings.

Normal operation of the downhole tool 610 is as described above inrespect of the downhole tool 10.

Referring now to FIG. 9, this schematically depicts a longitudinalelevation of a downhole assembly 700 in accordance with the presentinvention. The assembly 700 comprises a downhole motor 702 having amotor housing 704 and a rotatable motor output shaft 706. The motorshaft 706 is coupled through a first downhole tool 708, a drill collar710 (only the ends of which are shown), and a second downhole tool 712to a drill bit 714.

Each of the tools 708 and 712 is similar to the previously describeddownhole tools 10, 410, & 610 in having three skew-axis rollers mountedaround its periphery to provide radial support for the downhole assembly700, and to translate rotary motion during use of the assembly 700 intoa longitudinal force acting on the drill bit 714 to increase itseffective weight-on bit.

The motor housing 704 is coupled to and radially supported by a rollerassembly 716 having a peripheral array of rollers each having theirrotation axis tangential to a notional circle coaxial with thelongitudinal axis of the assembly 700 (equivalent to one of thepreviously described downhole tools but with a skew angle of 90°, orsomewhat like the outer part of the "antifriction bearing" of U.S. Pat.No. 1,913,365). The effect of the roller assembly 716 is to providecountertorque for the motor 702, i.e., to inhibit anticlockwise rotationof the motor housing 704 while the motor output shaft 706 is beingdriven clockwise by operation of the motor 702. This countertorque isachieved by the circumferential alignment of the roller axes in theroller assembly 716, which prevents free rotation of the roller assembly716 (though some limited rotation may take place due to slippage),though longitudinal movement of the roller assembly 716, and hence ofthe downhole assembly 700, can take place relatively freely.

The motor 702 is a hydraulic motor of the Moineau type which is fed withpressurised hydraulic fluid through a flexible tube 718 of the typeknown as "coiled tubing". The tube 718 is linked to the downholeassembly 700 through a fluid-tight rotary swivel 720 to prevent rotationof the motor casing 704 (due to slippage of the roller assembly 716)inducing undesirable distortions in the tube 718.

Turning now to FIG. 10, this shows a downhole assembly 800 which issimilar in many aspects to the above-described assembly 700, but whichdiffers in one substantive respect (detailed below). Those parts of theassembly 800 which are identical to or equivalent to like parts of theassembly 700 are given the same reference numeral, but with the leading"7" substituted by an "8" Therefore, for a full description of any partof the assembly 800 not detailed below, reference should be made to theappropriate part of the foregoing description of the assembly 700.

The substantive difference in the downhole assembly 800 with respect tothe downhole assembly 700 consists in replacing the roller assembly 716with a further downhole tool 830 which is essentially similar to thedownhole tools 808 and 812, except that the hand of the notional helixis reversed, i.e. each roller 832 is mounted on a respective roller axle834 which is tangential to a notional helix substantially coaxial withthe longitudinal axis of the tool 830 and spiralling clockwise ("righthand") in a downward direction (right to left as viewed in FIG. 10). Theeffect of this roller pitch reversal in the tool 830 with respect to theanticlockwise ("left hand") roller pitch in the tools 808 and 812 isthat as the motor housing 804 contrarotates (anticlockwise as viewedfrom above) as a consequence of reacting the clockwise output torque ofthe motor output shaft 806, the tool 830 produces a longitudinal forceacting in a downward direction (right to left as viewed in FIG. 10),thus dynamically adding to the effective "weight" on the drill bit 814.

The tool 830 is preferably set up and adjusted so that the tool 830 isless susceptible to longitudinal slippage than the tools 808 and 812. Aswell as the adoption of slippage-reducing measures such as providing therollers 832 with high-grip surfaces, such an objective can be attainedby additionally or alternatively urging the rollers 832 radiallyoutwards of the tool 830, e.g. by mounting the roller axles 834 onsprings (not shown) arranged to force the axles 834, and the rollers 832mounted thereon, radially outwards of the tool 830; alternatively theaxles 834 could be mounted on pressurizable actuators (not shown), e.g.hydraulic piston and cylinder assemblies, disposed to force the axles834 and the rollers 832 thereon radially outwards of the tool 830 whensuitably pressurised. Spring enhancement of roller traction forces (i.e.radial outward forces) has the advantage of being continuous andautomatic, while hydraulic or other pressure enhancement of rollertraction forces is capable of being suitably controlled in respect offactors such as timing and magnitude, thus enabling better performanceof the downhole assembly 800 in operation thereof.

Dominance by the tool 830 over the tools 808 and 812 in terms of theirrespective contributions to the production of longitudinal forces in acommon downhole direction can be further assured by making the tools 808and 812 undergauge, i.e. by arranging their roller axle locations and/orthe roller diameters to make the overall outside diameter of the tools808 and 812 marginally less than the bore of the previously drilled holein which the downhole assembly 800 is operated.

The tools 808 and 812 not only function to provide a dynamicallyincreased weight-on-bit (as previously detailed), the tools 808 and 812additionally function as stabilisers, i.e. they function to provideradial support for the parts of the downhole assembly 800 between andincluding the motor shaft 806 and the drill bit 814, allowing relativelylow-friction rotation of these components by reason of the rollersforming the peripheries of the tools 808 and 812. Thus the dual-functiontools 808 and 812 may conveniently be termed "traction stabilisers".Similarly, the tool 830 can be termed the "dominating stabiliser".

In the FIG. 10 arrangement, the negative effects of the reaction torqueof the motor 802 will be utilized to positive effect, providing anadditional thrust or motive force to that of the traction stabilisers808 and 812.

As the motor output shaft 806 rotates providing torque to the drill bit814, the traction stabilizers 808 and 812 provide forward thrust due totheir ability to "walk" into the wellbore under the influence of theleft-hand flutes incorporating the tractive rolling elements. The pitchof the left-hand helix will be constructed in such a way that thetraction stabilizers 808 and 812 will attempt to "walk" into thewellbore faster than either the coil-tubing 818 can be unreeled into thewellbore, or the drill bit 814 can cut into fresh formation. Thissituation creates slippage between the traction stabilizers 808, 812 andthe wellbore.

However, although the motor 802 will provide nominally constant rpm tothe drilling assembly, the fact that the dominating stabilizer 830 isconfigured to reduce the opportunity for slippage will cause a change inthe relative rotational speeds of the motor rotor 806 and motor casing804 with respect to the wellbore. It is envisaged that the motor casing804 will slow down in direct proportion to the reduction in forwardmotion from the calculated on the basis of the helix angle. The reducedrotational speed of the motor casing 804 will be compensated by anincrease in the rotational speed of the rotor 806, thereby providing thesame thrust to the drill bit 814, irrespective of the rotationalfluctuations of the assembly 800. In short, this system will provideautomatic compensation of the weight-on-bit longitudinal thrust providedat the drill bit 814.

To illustrate more fully and clearly the mechanism of operation thefollowing numerical illustration is shown by way of example, althoughthe figures given are not mandatory in every case.

Given that the best operation of typical coil-tubing is RIH ("run intohole") @1000 ft/hr it is imperative that the motive force provided bythe traction stabilizers is configured for significantly morelongitudinal progress than this.

1000 ft/hr=0.28 ft/sec

5 miles/hr=7.33 ft/sec

In effect this means that the traction stabilizers would "walk" downholeat 7.33 ft/sec but are constrained to 0.28 ft/sec, roughly 4% of theircapability. The remaining capability must therefore be dissipated asslippage between the traction stabilizers and the wall of the wellbore.

If the motor 802 is designed to operate at 400 rpm, and uses 300 rpm todrive the rotor 806 (and therefore the traction stabilizers 808 and 812)the remaining 100 rpm would be seen at the motor casing/dominatingstabilizer interface.

Given that the dominating stabilizer 830 will not slip, the rotationalspeed of the motor casing 804 will reduce from 100 rpm to 4 rpm, tocompensate for the reduction in forward motion of the stabilizers 808and 812, in direct proportion. Equally, the remaining 96 rpm will nowtransfer to the motor's rotor 806, and its shaft speed can betransferred back and forth between the rotor 806 and the casing 804 toprovide a constant thrust to the drill bit 814.

It is possible that due to the very shallow angles involved in thesetting of the left-hand stabilizers 808 and 812 that a mechanism can bedeveloped which inverts the orientation of the flutes and hence thehelix angle of the rollers such that for a continued input rotation thedownhole assembly would now "walk" back out of the hole.

Referring now to FIG. 11, this schematically illustrates a downholeassembly 900 which is a modification of the assembly 800 described abovewith reference to FIG. 10. The assembly 900 is configured to function asa pipe crawler or pipe tug assembly capable of pulling pipes, cables,inspection and testing equipment, and the like along tunnels, conduits,and similar underground passages that have been formed prior to thepassage of the assembly 900. Those parts of the assembly 900 whichcorrespond to equivalent or analogous parts of the assembly 800 aregiven the same reference numeral, but with the leading "S" replaced by a"9"; reference should be made to the appropriate parts of the precedingdescription for details of any part of the assembly 900 not describedbelow.

In the assembly 900, items forward (downhole or leftwards as viewed inFIG. 11) of the tool/stabilizer 908 are removed and replaced by abull-nose 940. The rear or uphole end of the assembly 900 is fitted witha cable attachment 950 to which (for example) a cable 960 may beattached to be dragged through the bore 970 by means of the assembly900.

The motor 902 would drive the traction stabiliser 908 which would "walk"along the pipe or conduit 970. The dominating stabilizer 930 would beconfigured to drag the cable 960 behind is as the assembly 900 rotatedand moved along the pipe 970. To obviate the difficulties encountered ata bend in the pipe 970 it is envisaged that the pipe tug assembly 900would have a universal coupling 980 (e.g. a Hooke joint) between themotor 902 and the traction stabiliser 908, thereby enabling the assembly900 to negotiate bends until limited by radii smaller than the longestsection length of the pipe tug. assembly 900.

It is also preferred that the aforementioned mechanism to reverse thehelix angle of the tractive elements 908 and 930 is included in theassembly 900. This would enable the traction stabilizer to "walk" out ofthe pipe for the same given rotation.

FIGS. 12-14 show a downhole drilling assembly 1000 essentially similarto the downhole assembly 80 of Fig. 10, but in more detail and somewhatless schematically. Parts of the assembly 1000 which directly correspondto parts of the assembly 800 are given the same reference numerals, butwith the leading "8" replaced by "10" (e.g., in FIG. 14, the motor whichis equivalent to the motor 802 of FIG. 10 is denoted "1002"). For adetailed description of the parts of the assembly 1000 and theiroperation, reference should be made to the foregoing description of theequivalent parts of the assembly 800 and their operation.

FIG. 12 is an elevational view of either one of the mutually identicaldownhole tools or traction stabilizers 1008 and 1012, while FIG. 13 is aplan view from above of the traction stabilisers 1008, 1012 (i.e. a viewfrom the left in FIG. 14 wherein the assembly 1000 is oppositelyoriented to the assembly 800 as depicted in FIG. 10). FIG. 14 is anelevation of the assembly 1000 drilling through geological material 1099(in a direction from left to right as viewed in FIG. 14). Operation ofthe assembly 1000 and of its constituent parts is as previouslydescribed in respect of the assembly 800 (FIG. 10).

FIGS. 15 and 16 illustrate a. downhole tool which is a variation on thepreviously described downhole tools. FIG. 15 is a longitudinal elevationof the outline of the tool 1100 in an operational position within thetubular casing 1190, while FIG. 16 is a longitudinal section of the tool1100 taken on a plane which is vertical to the center line of FIG. 15,and viewed in a direction which is right to left in FIG. 15.

In the previously described downhole tools, the rollers or other rollingelements had individual diameters which were small relative to theoverall peripheral diameter of the tool. However, the tool 1100 differsin that the rolling elements (detailed below) have individual diameterswhich are more nearly equal to (though still less than) the overallperipheral diameter of the tool.

Referring specifically to FIG. 16, the tool 1100 comprises a tubularcentral member 1102 upon which are mounted two spaced-apart single-rowball bearings 1104 and 1106 each fitted with respective toughened tyre1108, 1110 formed of metal, polymer, or any other suitable material.

Each of the bearings 1104 and 1106 is mounted on a respective tiltbearing 1112 and 1114 whose mutually parallel rotational axes are eachdiametrically aligned with respect to the longitudinal axis of thecentral member 1102. The bearing 1104 and 1106 are coupled by means (notshown) for controllable conjoint tilting in parallel planes about theirrespective tilt bearings 1112, 1114 such that each of the bearings 1104,1106 rotates about a respective axis which is angularly skewed withrespect to the longitudinal axis of the central member 1102. Theserotation axes of the bearings 1104 and 1106 are also laterally offsetfrom the longitudinal axis, in a direction which is upwards from theplane of FIG. 16, and rightwards in FIG. 15.

Between the mutually longitudinally spaced-apart bearings 1104 and 1106,the central member 1102 mounts a cluster of three mutually coaxialbearings 1116, 1118, and 1120 each dimensionally identical to thebearings 1104 & 1106, and each likewise being fitted with a respectivetoughened tyre. Each of the ball bearing 1116, 1118 and 1120 rotatesabout the same rotation axis which is parallel to the longitudinal axisof the central member 1102 (i.e. rotation axis is non-skewed), andlaterally offset equally and oppositely to the lateral offset of therotation axes of the bearings 1104 and 1106, i.e. the common rotationaxis of the bearings 1116, 1118, and 1120 is displaced in a directionwhich is downwards from the plane of FIG. 16, and leftwards in FIG. 15.

Thus the bearing pair 1104, 1106, and the bearing triplet 1116-1120contact mutually opposite sides of the casing 1190, as most clearlyshown in FIG. 15, thus to provide mutually opposed radial forces causingthese bearing groups each to bear against the inner face of the casing1190. The skew angle of the bearing pair 1104 and 1106 results in alongitudinal force being developed as the tool 1100 rotates within thecasing 1190, this longitudinal force being directed upwards as viewed inFIGS. 15 and 16 when the direction of rotation is clockwise as viewedfrom above and looking downwards.

FIG. 17 is a perspective view of a downhole tool 1200 based on the"large roller" principle described above with reference to FIGS. 15 and16. In the tool 1200, a central tubular member 1202 rotatably mountsupper and lower rollers 1204 and 1206 on respective rotation axes whichare angularly skewed with respect to and laterally offset from thelongitudinal axis of the tool 1200, as described above in respect of therollers 1104 and 1106 in the downhole tool 1100 of FIGS. 15 and 16. Thecentral member 1202 also rotatably mounts a central roller 1208 on arespective rotation axis which is laterally offset from the longitudinalaxis of the tool 1200 by an amount equal to and in a direction oppositeto the lateral offset of the rotation axes of the upper and lowerrollers 1204 and 1206. The rotation axis of the central roller 1208 maybe parallel to the longitudinal axis, or it may be skewed to match theskew of the rotation axes of the upper and lower rollers 1204 and 1206.Means (not shown) may be incorporated into the tool 1200 to cause therollers 1204, 1206, and 1208 to be mechanically and/or hydraulicallyurged radially outwards in a controlled or uncontrolled manner againstthe bore of the casing or other tubular cavity within which the tool1200 is being operated. Further means (not shown) may be incorporatedinto the tool 1200 for controllably varying the skew angles of therollers. The rollers 1204, 1206 and 1208 preferably incorporateperipheral inserts 1210 of a hard wear-resistant material (e.g.tungesten carbide), the rollers thereby superficially resembling`slices` of a conventional hard-faced fixed-blade stabiliser.

FIGS. 18 and 19 are respectively a schematic elevation and an end viewillustrating a developed form of a "large roller" downhole tool based onthe above described principles. In the downhole tool 1300 asschematically depicted in FIG. 18, a longitudinally extending centralmember 1302 mounts six large diameter rollers 1304, 1306, 1308, 1310,1312, and 1314 at spaced-apart locations along the central member 1302.Each of the rollers 1304-1314 has a respective rotation axis which isboth laterally offset and angularly skewed with respect to thelongitudinal axis of the central member 1302, i.e. the center line ofthe tool 1300, as depicted in FIG. 19. As shown in FIG. 18, the rollers1304-1314 have equal increments of mutual angular displacement of theirrespective lateral offsets, but this is not actually essential, therequirement being that the lateral offsets be angularly distributed inthe tool as a whole such as to provide a net balance of radial forces,i.e. such that a force in any one radial direction is balanced by adiametrically opposed radial force (or resultant of two or more radialforces).

Each of the rollers 1304-1314 contacts the surrounding casing 1390 at arespective point of contact (labelled "1"-"6" in FIG. 18) at which thecircumference of the respective roller makes a small angle (equal to theskew angle) with respect to a purely circumferential direction aroundthe bore of the casing 1390 at that point, such that if the tool 1300rolled around inside the casing 1390 without slipping, these points ofcontact would trace out paths equivalent to a screw-thread around andalong the base of the casing. Thus at the same time as the tool 1300provides rotational support for a downhole assembly of which it formspart, rotation of the tool 1300 tends to develop a longitudinal forcedriving the tool along the casing.

FIG. 20 (elevation) and FIG. 21 (plan) schematically depict a downholetool 1400 which is a modification of the tool 1300 described above withreference to FIGS. 18 and 19. In FIGS. 20 and 21, these parts of themodified tool 1400, which are equivalent or analogous to parts of thetool 1300 are given the same reference numerals, but with the leading"13" replaced by a "14"; for a description of any part of the tool 1400not detailed below, reference should be made to the relevant part of thepreceding description of the tool 1300.

In the tool 1400, the central roller-mounting 1402 has the general formof a helix, each of the rollers 1404-1414 being centrally mounted on thehelical member 1402 such that the required combination of lateral offsetand skew angle for each of the rollers 1404-1414 is provided by thehelical displacement of the member 1402 from the longitudinal axis ofthe tool 1400, rather than by offsetting the individual mounting of eachroller as in the FIG. 19 arrangement. The tool 1400 functions in thesame manner as does the tool 1300.

While certain modifications and variations of the invention have beendescribed above, the invention is not restricted thereto, and othermodifications and variations can be adopted without departing from thescope of the invention as defined in the appended claims.

We claim:
 1. A rotatable downhole assembly for rotatable operationwithin a previously drilled hole of substantially uniform diameter, saiddownhole assembly comprising a downhole motor having a motor housing anda rotatable motor output shaft coupled to a rotatable motor outpututilisation means, said downhole assembly further comprising at leastone downhole tool for providing radial support for a rotatable downholeassembly within a previously drilled hole of substantially uniformdiameter, said tool comprising a central member constructed or adaptedto be incorporated in a rotatable downhole assembly for rotationtherewith in use of said tool, said central member mounting a pluralityof non-reaming, non-cutting rolling element means in respectivepositions which are circumferentially distributed around said tool, eachsaid rolling element means being rotatably mounted on a respective axiswhich is tangential to a notional helix substantially coaxial with thelongitudinal axis of said tool about which said tool rotates in use ofsaid tool such that each said respective axis of said rolling elementmeans is skewed with respect to said longitudinal axis, each saidrolling element means having a respective periphery which extendssubstantially along the radially outermost periphery of said toolwhereby the radially outermost periphery of said tool provides rollingradial support for said rotatable downhole assembly in use of said toolby means of the peripheries of said rolling element means and therotation of said rolling element means about their skewed axestranslates rotation of said tool in use thereof to alongitudinally-directed force acting through said central member on saiddownhole assembly, said at least one downhole tool being coupled betweensaid rotatable motor output shaft and said rotatable motor outpututilisation means for rotation therewith in operation of said assemblyto provide radial support therefor and to translate such rotation to alongitudinally-directed force acting through said motor outpututilisation means, wherein the motor of said downhole assembly is ahydraulic motor supplied in operation thereof with pressurised fluid byway of tubing, said downhole assembly being coupled to said tubing byway of a swivel coupling which is substantially fluid-tight.
 2. Adownhole tool as claimed in claim 1 wherein said rotatable downholeassembly is a drillstring and said notional helix is contra-rotary withrespect to the combination of the normal or forward direction ofrotation of the drillstring and the direction from said tool towards adrill bit at the downhole end of the drillstring, whereby normal orforward rotation of said drillstring and of the tool incorporatedtherein results in a longitudinal force tending to propel thedrillstring towards the blind end of the bore and ultimately tending toforce the drill bit into the geological material to be drilled.
 3. Adownhole tool as claimed in claim 2 wherein said normal or forwarddirection of rotation of the drillstring is clockwise as viewed from thesurface and looking down into the bore and said notional helixprogresses anti-clockwise in a downhole direction therealong whereby theperipheries of said rolling element means, where they extend to theradially outermost periphery of the tool, align with a notionalright-hand thread around the outer periphery of said tool.
 4. A downholetool as claimed in claim 1 wherein each respective axis of said rollingelement means is skewed with respect to the longitudinal axis of thetool at an angle in the range from a very low (but non-zero) angle, upto 45°.
 5. A downhole tool as claimed in claim 1, wherein said rollingelement means are rollers, and the peripheries of said rollers areindividually cylindrical or crowned.
 6. A downhole tool as claimed inclaim 5 wherein said rollers are individually mounted on a respectiveaxis.
 7. A downhole tool as claimed in claim 5, wherein said rollers aremounted in coaxial groups, such that within a group of rollers,individual rollers of that group are capable of rotating at mutuallydiffering rotational rates.
 8. A downhole assembly as claimed in claim1, wherein said downhole assembly comprises a plurality of such downholetools, and each being coupled between said rotatable motor output shaftand said rotatable motor output utilization means.
 9. A downholeassembly as claimed in claim 1, wherein said rotatable motor outpututilisation means comprises a drill bit, said at least one downhole toolcomprised in said downhole assembly being arranged to increase theeffective weight-on-bit during normally directed rotation of said drillbit by said downhole motor.
 10. A downhole assembly as claimed in claim1, wherein said motor housing is coupled to countertorque means forreacting motor torque output by said motor output shaft, saidcountertorque means rotationally constraining said motor housing withrespect to said previously drilled hole.
 11. downhole assembly asclaimed in claim 10, wherein said countertorque means provides arotational braking effect while allowing relative freedom of movement ina longitudinal direction.
 12. A downhole assembly as claimed in claim11, wherein said rotational braking effect is provided by forming saidcountertorque means with a peripheral array of hole-contacting rotatablerollers having their axes of rotation substantially tangential tonotional circles substantially coaxial with the longitudinal axis ofsaid downhole assembly.
 13. A downhole assembly as claimed in claim 11,wherein said countertorque means comprises a further downhole tool, thenotional helix of said further downhole tool being oppositely handedwith respect to the notional helix of said at least one downhole toolcoupled between said rotatable motor output shaft and said rotatablemotor output utilisation means whereby relative contrarotation of saidmotor housing with respect to said motor output shaft results incommonly directed longitudinal forces at said at least one and furtherdownhole tools comprised in said downhole assembly.
 14. A downholeassembly as claimed in claim 12, wherein said downhole assembly hasmajor components and sub-assemblies thereof longitudinally coupled byone or more couplings transmissive of torque and longitudinal forces butyieldable about axes transverse to the longitudinal axis whereby thedownhole assembly may conform to bent holes.
 15. A downhole tool asclaimed in claim 4, wherein said angle is in the range from 0.5° to 15°.